Strawberry planting substrates

In my initial experience trying to plant strawberries, I had lots of issues which I put down to the substrate. But it's hard to be sure what exactly it is about one or other substrate, that makes it suitable or not. Added to the problem is that many substrates that you get locally do not label the nutrient and other characteristics of the substrate (eg coarseness, salinity, conductivity, besides the usual NPK content).

So I looked up strawberry substrates on Google, and came across this one by Plantaflor.

What's interesting are the stated properties, which I will use in future for reference.

Coarse structure, with a relatively low salt concentration (KCL 0.7g/l), NPK 18-10-20.

More recently, by chance I bought some Bioflora all purpose potting mix (with Sunflower on packet). This mix looks to be high quality with a uniform dark crumbly texture, and no random sticks or clay balls.

On the other hand, the Bioflora potting mix on the right with random flowers seems to be of lower quality, and I wonder if there is possibly disease in the substrate.

I potted one of my Korean strawberry runners with the Bioflora Sunflower mix and finally got it to flower!

Plant on the right is a Korean strawberry plant, grown from a runner from the mother plant which was grown from seed. The plant on the left is an Alpine strawberry plant from a Paris Garden kit.